Very complex cross database select statement

In my select statement below I need the ability to change the database I'm selecting from on a record by record basis.

So...when Left(p.afsSource,2) = 'LT'
I pull from proc_lt.dbo.P_PaymentStreams

Else
I pull from  crmprod_01.dbo.P_PaymentStreams


The top part will ALWAYS pull from proc_cfa.dbo.P_AvailableForSale

Declare @OrdTranID int  
Set @OrdTranID = 9001
Select  Left(p.afsSource,2) as busType,    
        SubString(p.afsSource,4,4) as DealID,  
		Sum(p.afsAMount) as afsAmount,
		st.cp as CompoundingMethod         
from    proc_cfa.dbo.P_AvailableForSale p   
        LEFT JOIN (  
                Select              
                        ps.DealID    
                        ,CASE WHEN COUNT(ps.PmtFrequency) > 1 THEN 'Split' ELSE MIN(ps.PmtFrequency) END cp    
                        ,MAX(ps.loanDate) LoanDate  
                from    proc_lt.dbo.P_PaymentStreams ps  
                Group by ps.DealID   
        ) st ON st.DealId = SubString(p.afsSource,4,4)  
where   p.afsTransAction = @OrdTranID    
group by Left(p.afsSource,2), SubString(p.afsSource,4,4), st.cp

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